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Tensile-testing video extensometers

In a previous RET-Monitor we looked at testing the material properties of composites, so this month we will investigate a machine that is particularly suited to tensile testing of such materials, the video extensometer, which can provide an accurate measurement of deformation and failure while also providing invaluable visual data on the method of failure.

Material stress is normally calculated by measuring the specimen’s cross-sectional area and relating this to the measured load obtained from a calibrated load cell while loading the specimen via grips or adapters. The problem with this type of testing though is that it can be difficult to test some samples, particularly those that fail with a lot of force, making it hard to asses dimensional changes accurately or those that need to be tested over a large range, where travel limits on strain gauges can be an issue. It is here that the non-contact nature of a video extensometer can prove effective, as the method can provide measurements up to and including the point of failure, as well as accommodate large or oddly shaped samples.

Method of operation

A video extensometer captures a continuous image of the specimen under test, using a rig fitted with a high-frame-rate digital camera placed in a fixed location to the specimen. The specimen is marked with two indicator points that contrast with the base material, allowing them to be recognised by post-processing software. It is essential that the markers are as sharp and as different in contrast as possible, to ensure correct automatic target recognition and tracking. The target position is detected at the edge of a contrast transition and is hence not affected by changes in target width.

As the material is elongated, the software measures the pixel distance between the two markers and compares it to a calibration value to provide a strain measurement. The calibration value is obtained by first testing a calibration specimen that has known properties and often accurately etched positional markers. A correctly calibrated video extensometer should be capable of recording at a resolution of 1 µm.

One factor that can affect test results from this system is changes in ambient lighting conditions. If the lighting changes then the contrast between the markers and the test specimen can also change, impacting accuracy. To combat this, filters are used on both the lights used to illuminate the subject and the video capture lens, removing any possible variations.

Beyond simple strain measurements, some video extensometers can also be configured to provide strain distribution analysis. This is achieved by applying multiple markings to the surface of the test specimen and tracking the positional change of these marks using processing software. This information can then be used to ascertain the level of deformation as load is applied, and the nature of the deformation – a useful feature where the dimensional properties of a part under load are important.

Video extensometers are a proven technology and can be very useful where standard testing methods are impractical. They can also be a very cost-effective solution, as systems are available for retrofitting to more traditional extensometers, providing the option of both contact and non-contact measurement in the same machine.

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